1. Field of the Invention
The present invention relates to production of ensemble and anharmonic overtones in a polyphonic tone synthesizer.
2. Description of the Prior Art
An ensemble effect is produced in a pipe organ by using two or more ranks of pipes, one of which is tuned to the nominally correct eight-foot frequency while another is slightly out-of-tune. When a single keyboard switch is depressed, both the in-tune and out-of-tune pipes are sounded. The resultant beat frequencies are most pleasant to the listener. An ensemble effect using two sets of pipes having the same or essentially similar tonal quality is known as "doubling." More complex effects are achieved using three or more sets of pipes and thereby more closely approaching the ensemble effect of a full orchestra.
In conventional electronic organs an ensemble effect is obtained by using separate sets of oscillators offset in frequency with respect to the nominal in-tune tone generators. When combined electronically or acoustically, the combined generator outputs produce an ensemble effect. Alternatively, two separate and complete electronic organ systems, detuned with respect to each other and used to drive different speakers, may be used to produce ensemble. Such approaches are expensive and virtually rule out ensemble effects more complex than two out-of-tune systems. Deutsch, in U.S. Pat. No. 3,884,108 discloses a system for producing ensemble in a Computer Organ such as that described by Deutsch in U.S. Pat. No. 3,809,786. Therein is disclosed apparatus for producing an ensemble effect in a computor organ of the type wherein muscial notes are generated by computing the amplitudes at successive sample points of a musical waveshape and converting the amplitudes to sounds as the computations are carried out in real time. Each amplitude is computed during a regular time interval t.sub.x by separately calculating a set of "combined" Fourier components which are accumulated to obtain the sample point amplitude. Each n.sup.th order combined Fourier component is evaluated by summing a pair of sinusoid values associated respectively with the nominal tone and the frequency offset, "out-of-tune" tone. The sum is multiplied by a harmonic coefficient to establish the relative amplitude of that combined Fourier component.
Deutsch, in U.S. Pat. No. 3,888,153 discloses a system for producing tones having anharmonic overtones in a Computer Organ (U.S. Pat. No. 3,809,786). Therein is disclosed apparatus for creating, in real time, tones wherein the fundamental frequency is at the true nominal pitch but the overtones are displaced from their true frequencies. This frequency displacement is such that if an amount d is used for the second harmonic, then 2d is used for the third harmonic; and (n-1)d is used for the n.sup.th harmonic. The means disclosed in U.s. Pat. No. 3,888,153 for producing anharmonic overtones in a Computor Organ are essentially similar to the means disclosed in U.S. Pat. No. 3,884,108 for producing ensemble effects in a Computor Organ. The principle difference being that for anharmonic tone generation the Fourier components corresponding to the fundamental frequency are computed for the true pitch while only the overtones are computed out-of-tune.
An object of the present invention is to provide a single system for producing ensemble effects in combination with anharmonic overtones in a polyphonic tone synthesizer. The subsystem add-on to a polyphonic tone synthesizer is inexpensive and provides tonal effects heretofore only available on large expensive electronic organs.